1. Field of the Invention
The present invention relates to a device for controlling steering characteristics (i.e. handling characteristics) of a vehicle such as an automobile, and more specifically, to such a device that suppresses disturbances in the behavior or posture of the vehicle due to differences between driving and braking forces on the left and right wheels.
2. Description of Prior Art
In a modern power steering system equipped on a vehicle, a steering control device controls steering torque, applied to steered wheels, based upon parameters of a running condition of the vehicle, such as a vehicle speed. So far, in addition to the purpose of the reduction of a force required for rotating the steering wheel of the driver of the vehicle, steering control devices are variously designed so as to provide comfortableness and safety in driving a vehicle for the driver. Some steering control devices serve for assisting the driver of a vehicle in tracking a course of travel of the vehicle while keeping a stable running behavior or posture of the vehicle. For instance, European Patent No. 0 296 756 describes a steering control device, which adjusts a force produced by, and/or a ratio of, a steering gear as a function of differential rotational velocity of two or more wheels and a position of a steering gear, providing enhanced handling in events that the left and right wheels each have different conditions. Japanese Patent Laid-Open Publication No. 2001-334947 shows a steering control system operating in response to a difference between the left and right wheel velocities, in which system, upon braking the vehicle, the angle of a steering gear is automatically biased toward the side of the smaller wheel velocity. Japanese Patent No. 2861561 and Japanese Patent Laid-Open Publication No. 2001-80535 each describe a control device adjusting steering assist torque based upon braking and/or traction forces generated on the left and right wheels. The above-described control devices, in general, compensate, by imparting steering assist torque and/or controlling the steering angle of a vehicle, for imbalance of forces of different magnitudes generated on individual wheels or tires under the control of Traction control (TRC) device and/or Anti-skid control (ABS) device, such as when the vehicle is running on a road surface having different frictional coefficients, e.g. dried in one side and wet or frozen in the other side.
Steering control devices in the prior art, taking into account the force balance between left and right wheels, can effectively assist in suppressing disturbance in the posture of a vehicle due to the force imbalance when the vehicle is running along a straight course. Under the control of those steering control devices, steering assist torque is produced or increased in a direction against a yaw moment induced on a vehicle body by a difference between forces on the left and right wheels. That is, when a vehicle is accelerated (decelerated) on a road surface having different frictional coefficients on which a yaw moment induced by traction (braking) forces on the wheels tends to turn the vehicle body to the lower (higher) frictional side, the steering assist torque will be given or increased in the direction suppressing the turning of the vehicle body. Alternatively, the steering angle of the vehicle is directly controlled so as to generate a yaw moment for maintaining a stable posture of the vehicle against a yaw moment induced by the traction or braking forces on the wheels. Accordingly, the steering assist torque (and/or the steering angle control) will lighten the burden of steering his handle, imposed on the driver, for stabilizing the vehicle posture and keeping the straight course of travel of the vehicle.
However, the steering assist torque and steering angle control (Steering Assist Control) as described above do not always support the driver's steering operation: it would deteriorate the steering characteristic of a vehicle when the vehicle is turning or running in a curved course. During turning of a vehicle, because of the centrifugal force, vertical loads become higher on the turning outer wheels, so that the road reaction (traction and braking) forces on the turning outer wheels become higher than those on the turning inner wheels. This distribution of the road reaction forces is similar to that in a vehicle running on the surface having different frictional coefficients: the forces on the wheels in one side are higher than in the other side. Thus, the steering control devices in the prior art produce or increase the steering assist torque or the steering angle in the direction of suppressing yaw moment induced by the road reaction forces even on a road surface of a uniform frictional characteristic.
During turning of a vehicle, a yaw moment turning the vehicle head along the curved course, i.e. from the turning outside to the turning inside is required. In the prior art, however, the steering assist torque in accelerating the vehicle is generated in the direction from the turning inside to the turning outside, opposite to the required yaw moment, because the traction forces on the wheels are higher in the turning outside. Thus, the steering assist torque produced or increased under the control of the prior device increases the force required for rotating the steering wheel, leading to a failure of steering of the driver and undesirable increase in the turning radius due to the reduction of the steering angle (Deterioration of the turning ability of the vehicle). On the other hand, the steering assist torque during decelerating of the vehicle under the control of the prior device is generated or increased in the same direction as the required yaw moment for turning the vehicle body. Because of the addition or increase of the steering assist torque to the total steering torque, however, the force for rotating the steering wheel is made smaller unexpectedly for the driver, which could induce an excessive rotation of the steering wheel, leading to the undesirable reduction of the turning radius (Reduction of the turning stability of the vehicle). Similar phenomena would occur under the steering angle control in the prior art. In other words, the steering assist control in the prior art is unexpectedly operated to bias the steering torque and/or steering angle during turning of a vehicle, because it is basically based upon only the driving and braking forces on the wheels.
In a steering assist control, undesirable and unexpected assist operation modifying a steering characteristic of a vehicle should be avoided. During turning of a vehicle in a curved course, especially, the driver should feel an appropriate load or force in turning his vehicle in order to prevent failing to track the driving course. Thus, the conventional steering control devices may be improved more appropriately.